Instrument for Continuous High Resolution Measurement of Changes in the Velocity of Ultrasound

Abstract

The frequency of the ``reference'' oscillator in a coherent pulse echo apparatus is phase‐locked to the spacing in time of the acoustic echoes. A change in echo spacing is thereby converted to the same fractional change of oscillator frequency, which is measured continuously. Echo spacing changes of 2 to 3 parts in 10⁸ have been resolved at 12 Mc. The gated AFC circuit which controls the reference oscillator derives its error voltage from a phase‐detected echo. The error voltage is zero when the echo is in quadrature with the reference. When the echo spacing changes, the AFC shifts the reference frequency to preserve the quadrature condition. The system is largely insensitive to changes of echo amplitude because such changes do not affect the quadrature condition. The attainable accuracy is set mainly by the temperature stability of the sample. There is a detailed discussion of circuitry, performance, and suggested refinements.